Chemical and physical changes can be distinguished based on whether changes occur at the molecular level or involve energy and states of matter. Physical changes alter the shape or state of matter of a substance without changing its chemical composition, such as crushing a can or melting ice. Chemical changes involve transformations of molecular structure and composition through breaking and forming of chemical bonds, such as rusting of iron or burning of sugar. While some changes are obvious, others occur at the microscopic level of molecular structure and bonding.

1. Topic 1
Chemical vs. Physical
Changes
It is important to understand the difference
between chemical and physical changes. The two
types are based on studying chemical reactions
and states of matter. We admit that some
changes are obvious, but there are some basic
ideas you can use. Physical changes are about
energy and states of matter. Chemical changes
happen on a molecular level.
When you step on a can and crush it, you have
forced a
physical change. The shape of the object has
changed. It wasn't a change in the state of matter,
but something changed. When you melt an ice
cube you have also forced a physical change
(adding energy). That example caused a change
in the state of matter. You can cause physical
changes with forces like motion, temperature, and
pressure.
Chemical changes happen on a much smaller
scale. While some experiments show obvious
chemical changes such as a color change, most
chemical changes happen between molecules and are unseen. When iron (Fe) rusts
you can see it happen over a long period of time. The actual molecules have
changed their structure (the iron oxidized). Melting a sugar cube is a physical change
because the substance is still sugar. Burning a sugar cube is a chemical change.
The energy of the fire has broken down the chemical bonds.
Some changes are extremely small. Chemical changes can happen over a series of
steps, and the result might have the same number of atoms but have a different
structure. The sugars glucose, galactose, and fructose all have six carbon atoms,
twelve hydrogen atoms, and six oxygen atoms. Even though they are made of the
same atoms, they have very different shapes and are called structural isomers. They
each have different chemical reactions because of their molecular structure.
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